In solid polymer fuel cells which employ an ion exchange membrane (typically a proton exchange membrane) as the electrolyte, the water content of the membrane affects the performance of the fuel cell. The ion conductivity of the membrane generally increases as the water content or hydration of the membrane increases, therefore it is desirable to maintain a sufficiently high level of hydration in the membrane during fuel cell operation. For this reason, the reactant streams are typically humidified prior to introduction into electrochemically active regions of the fuel cell.
The capacity of reactant gases to absorb water vapor varies significantly with changes in temperature and pressure. Therefore, it is preferred to humidify the reactant gas streams at or as near as possible to the operating temperature and pressure within the fuel cell. If the reactant gas is humidified at a temperature higher than the fuel cell operating temperature this can result in condensation of liquid water occurring when the humidified reactant gas enters the fuel cell. Condensation may cause flooding in the electrodes which may detrimentally affect fuel cell performance. Conversely, if the reactant gas stream is humidified at a temperature lower than the fuel cell operating temperature, the reduced water vapor content in the reactant gas stream could result in membrane dehydration and damage to the membrane. Thus, the reactant streams are often heated and humidified prior to introduction into the fuel cell.
Various approaches have been used to increase the humidity of reactant gas streams supplied to fuel cells. For example, some conventional solid polymer fuel cell systems humidify a reactant gas stream by flowing the reactant gas stream and liquid water on the opposite sides of a water-permeable membrane. Water from the liquid stream is transferred through the membrane, thereby humidifying the reactant gas stream. The pressure, temperature, flow rates and path length through the liquid water-to-gas humidifier can be adjusted to give the desired water vapor content in the reactant stream. The water is preferably de-ionized to prevent ionic contamination of the fuel cell membrane electrolyte.
For example, U.S. Pat. No. 5,547,776 discloses a fuel cell system which includes a membrane humidifier for a reactant gas stream with the humidifier preferably using de-ionized liquid water.
Such liquid water-to-gas humidifiers are commonly used in solid polymer fuel cell systems in which water is used as a cooling fluid, as the cooling water is a convenient source of water for the humidifier. Japanese Patent Publication No. 09-055218, for instance, teaches membrane humidification of the reactant gas supply stream using the warm water of the liquid coolant. The humidifier employed may be a separate module in the solid polymer fuel cell system. Alternatively, it may be incorporated as an assembly in the fuel cell stack itself, such as between the end plates of fuel cell stack. U.S. Pat. No. 5,382,478 discloses such an assembly in the fuel cell stack wherein a liquid water-gas humidification section is located upstream of the electrochemically active section of the fuel cell. U.S. Pat. No. 4,973,530 shows another embodiment comprising a liquid water-gas humidification section. In fact, the humidification section can be even more closely associated with the electrochemically active section of the fuel cell stack. For instance, PCT International Publication No. WO 96/24958 shows that humidification sections can be created by leaving portions of the electrode surfaces uncoated with catalyst. Thus, humidification of the anode stream by the cathode stream can occur across the membrane electrolyte material in these areas. However, these uncoated portions are not electrochemically active and thus represent inefficient use of electrode and polymer electrolyte membrane surface area.
The reactant streams exiting the fuel cell or fuel cell stack typically contain product water, as well as water vapor which was present in the humidified stream delivered to the fuel cell. In particular, the oxidant stream, as it travels through a fuel cell, absorbs water that is produced as the product of the electrochemical reaction at the cathode.
In some fuel cell systems, the product water from the fuel cell is condensed from the exhaust reactant streams and is collected and then used for reactant stream humidification and/or cooling of the fuel cell. In such systems, the water in the exhaust streams is typically collected in the liquid phase and is generally combined with a larger liquid cooling water supply as shown, for instance, in U.S. Pat. No. 5,200,278.
Other conventional approaches for humidification of reactant gas streams prior to introduction into fuel cells include the injection of water vapor or atomized water droplets into the reactant streams (as shown, for instance, in U.S. Pat. No. 5,432,020), and exposing a reactant gas stream directly to water in an evaporation chamber to permit the stream to absorb evaporated water. Japanese Patent Publication No. 07-176313 shows a fuel cell system where pure water is used to humidify the reactant gas supply streams and where the oxidant stream exhaust is used to heat the incoming oxidant reactant gas stream in a heat exchanger.
Solid polymer fuel cell systems are typically liquid-cooled rather than air-cooled if higher power densities (power output capability per unit volume) are required. The reason is that their cooling systems must shed a significant amount of heat at relatively low temperature (circa 80.degree. C.) with respect to ambient temperature . In addition, the use of liquid--as opposed to air-cooling allows the fuel cell stack cooling channels to be made smaller and hence a lower overall stack volume can be obtained. However, air-cooled fuel cell systems may be preferred in many applications where power density is less important. However, for humidification purposes, such air-cooled systems cannot rely on the coolant as a supply of water.
It is well known that transport across a semi-permeable membrane is substantially more efficient from the liquid phase than from the gas phase. Nonetheless, it is also well known that a significant, but lesser, transfer of species can take place across a suitable semi-permeable membrane, from the gas phase, even against an absolute pressure differential, as long as there is a significant partial pressure difference of the species across the membrane. For instance, U.S. Pat. No. 3,494,174 discloses a semi-permeable membrane exchange apparatus for transferring species from one gas stream to another, against an absolute pressure difference, for use in gas chromatographs. However, predominantly gas--gas phase membrane exchange humidifiers have generally not been contemplated for use in humidifying the reactant gas supply streams of fuel cells. Presumably, the heating and humidity requirements for fuel cell reactant gas supply streams were not viewed as achievable in gas--gas humidifiers.
In fuel cell systems which employ conventional reactant gas supply humidifiers, the desired heating of the supply stream may be accomplished simultaneously with the humidifying. For instance, heat is exchanged across the membrane in any liquid water-gas membrane humidifier supplied with warm liquid water from the circulating coolant.
Sometimes it is not necessary to use a humidifier to introduce water vapor into both the oxidant and fuel reactant streams. In systems in which the reactant gas is partially or fully recirculated through the fuel cell stack, it may not be necessary to humidify the additional fresh gas stream which is also supplied, if the product water carried back to the inlet by the recycled stream is sufficient to maintain adequate hydration of the ion exchange membrane. U.S. Pat. No. 5,543,238 illustrates partial recirculation of the fuel cell exhaust gas to heat and humidify the incoming reactant stream. Additional apparatus for performing the recirculation is necessary however. In direct methanol fuel cells, the methanol fuel stream may be supplied as a dilute aqueous solution in which case further humidification of the fuel stream is not required. Similarly hydrogen-containing reformate streams (obtained, for example, by reforming a fuel such as methanol, natural gas or butane), which may be supplied as the fuel stream to a fuel cell, typically contain sufficient residual water vapor from the reforming process.
While many varied fuel cell systems appear in the art, typically at least the oxidant gas supply stream is humidified and heated prior to introduction into a fuel cell. The required humidification and heating apparatus typically adds to the complexity of the fuel cell system, as it generally includes additional system components, such as a humidification water pump, piping, water reservoir and filtration unit, in addition to the humidification module and heater. It can also add to the parasitic load of the system as power is required for operating pumps and heating the stream.
Accordingly, a simpler and more energy efficient means for pre-heating and humidifying reactant supply streams in a solid polymer fuel cell systems is desirable.